The invention relates to vacuum integrated circuits of the type having thermionic cathodes or cold cathodes, grid elements, and anodes operative in a vacuum, and more particularly to improvements therein that permit stable, reliable operation of a plurality of such devices within a single vacuum region.
Although semiconductor integrated circuits (ICs) are widely used and are highly reliable, and are very inexpensive, there are certain applications for which conventional semiconductor integrated circuits are poor suited. Generally, operating temperature ranges of semiconductor integrated circuits are between about -65.degree. C. and +200.degree. C. Although high power semiconductor devices are available, most semiconductor integrated circuits are limited to relatively low power applications. Many semiconductor devices have lower bandwidth than can be achieved with suitably designed vacuum integrated circuits. This is due to the low grid-cathode interelectrode capacitance. Bipolar and MOS integrated circuits become inoperative in the presence of large amounts of nuclear and electromagnetic radiation, as would occur in the event of nuclear explosions. It is estimated that most "silicon based" electronics would be destroyed by the electromagnetic pulses (EMP) produced by nuclear detonations. All of the common semiconductor integrated circuit manufacturing processes are highly refined, complex, inexpensive processes in which minor variations can result in great reductions in IC manufacturing yield. Minute defects in the semiconductor material also can result in costly reduction in IC manufacturing yields.
Consequently, there are certain applications in which there would be a good market for stable, reliable vacuum integrated circuits, if such could be economically manufactured. Therefore, considerable research and effort has been directed to developing a reliable, manfacturable vacuum integrated circuit, as indicated in U.S. Pat. Nos. 3,701,919, 3,978,364, 4,138,622, which are generally indicative of the state-of-the-art.
The device vacuum integrated circuit shown in U.S. Pat. No. 3,701,919 (Geppert) is a coplanar device. A great deal of capital and approximately three years of time were spent by Electron Emission Systems, a subsidiary of Baldwin Electronics Company of Hot Springs, Ark. in attempting to develop the disclosed structures, before the project failed. The structure shown in FIG. 3A of the Geppert reference discloses two cascaded triodes in a coplanar structure that is enclosed within a vacuum chamber. Reliable operation of one triode of the structure was achieved as a result of electron charge build-up on the inner walls of the glass vacuum envelope, but no success was achieved in providing more than one triode structure capable of stable, simultaneous operation in the same vacuum envelope. I believe that the failure of the Geppert devices to operate reliably was a result of not having the proper device elements for producing the necessary electric fields which would allow the anode to collect all the electrons emitted by the cathodes of the same device. At the same time in the Geppert devices there were extraneous electric fields induced as a result of build up on various interior surfaces of the glass vacuum envelope by electrons emitted from the cathodes. These electric fields repelled the electrons from the cathode of the single triode back to the anode where they are supposed to be collected. However, for the case of multiple triodes, the repelled electrons originating from a given device cathode are collected by the anodes of other devices. This cross-talk between devices severely degrades the desired circuit performance.
To bypass the problem of the Geppert structure, all subsequent device development has been in the area of biplanar structures (e.g., U.S. Pat. Nos. 3,978,364 and 4,138,622) wherein the anode is directly above the cathode and is able to collect a large percentage of electrons emitted by the cathode. The biplanar structure has the following disadvantages:
1. Two substrates have to be designed and fabricated using two different mask sets.
2. The substrates have to be accurately aligned. The device performance depends critically on the alignment. As the device geometry gets smaller, the alignment becomes even more critical and increasingly more difficult.
3. The distance between substrates is critical in determining the device characteristics and is difficult to control accurately over the entire surface of both substrates.
4. To make low voltage operating devices, the distance between substrates has to be made smaller. The smaller this distance is made, the more difficult it becomes to control it accurately. For thermionic cathodes using the emission carbonates, the two closely spaced substrates make pumping of the gases evolved during cathode activation difficult. The high pressure developed in the region between the two substrates could be potentially harmful to proper cathode activation.
5. Since the metal films on the substrates are thin films, it is difficult to make low and high voltage operating devices on the same substrate because it is difficult to make anodes which are at large varying distances from the cathodes.
All of these problems are avoided with the planar structure, since complete circuits are made on a single substrate and the dimensions of structures on the substrate are controleld by photolithographic techniques which are accurate to within one micron dimensions.
Thus, there remains an unmet need for an improved, reliable, reasonably inexpensive, and hence coplanar integrated circuit operable in a single vacuum chamber.